Cavitational method for drilling wells



June 1968 E. L. COOK 3,387,672

CAVITATIONAL METHOD FOR DRILLING WELLS Filed June 26, 1964 -PL (DRILLING FLUID TO ROTARY) PL- (GAS TO ROTARY) L P (DRILLING FLUID a GAS TO PITS) FIG. I

//2I /IO L 3 '43 ooooooo bh 5 4 Lu g 5 m m EVIN L. COOK w P bh INVENTOR. 5 p P BWWM ATTORNEY United States Patent O 3,337,672 CAVHATIGNAL METHGD FOR DRILLING WELLS Evin L. 600k, Dallas, Tom, assignor to Mobil Gil Corporation, a corporation of New York Filed June 26, 1964, Ser. No. 378,137 8 Claims. (Cl. 175-69) This invention relates to a method for drilling wells and is specifically applicable to the drilling of wells where some form of vibratory action occurs at the bottom of the borehole and it is desired to establish a state of cavitation at such bottom.

It is well known that where a state of cavitation exists within a body of liquid, certain destructive effects are realized along the surfaces of solid material exposed to the collapse of the cavities or bubbles. The destructive effects of cavitation are undesirable in situations such as the operation of ship propellers where cavities created in the water around the propeller due to rotation result in pitting the surfaces of the propeller. Extensive studies have been made regarding ways in which these destructive effects can be minimized. On the other hand, it has been proposed that conditions of cavitation may be intentionally established and taken advantage of for such purposes as drilling boreholes in the earth. The cavitational effect may be used to augment direct contact drilling where a drill bit is employed or it may be used as the primary force in advancing a borehole, such as where cavities are created within a stream of liquid directed against the bottom of the borehole. Where drilling is being primarily accomplished by a drill bit and is to be augmented by the cavitational effect, such drillin may be done with a tool of the type illustrated by Albert G. Bodine, Jr. in his US. Patent No. 2,554,005, entitled Earth Boring Apparatus. In conventional rotary drilling, the rolling of the toothed cones of the bit causes vibratory action of the lower end of the drill string assembly which may be utilized to establish a state of cavitation in accordance with the invention. A method of drilling where the formation is primarily contacted by only a stream of drilling fiuid in which a condition of cavitation is established is disclosed and claimed in a copending patent application by Warren B. Brooks, Ser. No. 203,076, filed June 18, 1962, now abandoned in favor of application Ser. No. 470,265, filed June 7, 1965. In the instance of each approach to the problem of utilizing the cavitational effect in drilling, the actual condition of cavitation is brought about by the reciprocation of an element of the drilling tool in the drilling liquid. As the element is rapidly retracted from the surface being penetrated, a reduction in pressure occurs with the result that, if proper conditions exist, a plurality of bubbles or cavities will be induced in the drilling liquid.

As a wellbore is continually deepened by drilling with the cavitational method, the ability to develop a state of cavitation progressively becomes more diflicult due to the hydrostatic pressure at the bottom. Depending, of course, upon the equipment being employed and the conditions under which drilling is being carried out, a depth will be reached at which it may become either impossible or at least impractical to cavitate the drilling liquid.

It is one object of the present invention to provide a method of drilling a borehole in an earth formation utilining the cavitational effect. It is another object of the present invention to provide a cavitational method of drilling a borehole which may be utilized at greater depths than the methods which previously have been suggested. These and still further objects of the invention will be evident from the following specification taken in conjunction with the accompanying drawings.

In accordance with the invention, an improved method "ice of cavitational drilling is provided by introducing into the drilling liquid 21 gas in an amount which will substantially saturate the drilling liquid at the bottom of the borehole at a pressure equivalent to the bottomhole pressure re duced by the magnitude of the acoustical pressure developed by the particular drilling apparatus employed. A drilling tool, either a conventional rotary type core bit or a special reciprocable type unit, is supported on a drill string and lowered to the bottom of the borehole. Drilling liquid is circulated to the bottom of the borehole through the drill string and back to the surface through the annulus between the drill string and the wall of the borehole. The drilling tool is actuated to develop a cyclical, reciprocating motion in the vicinity of the borehole bottom which results in alternating reductions and increases of pressure within the liquid below the element being reciprocated. If the pressure is reduced to a sufficient level, cavities will form within the liquid. The presence of the gas injected in accordance with the invention permits cavitation to occur at increased hydrostatic pressures.

In the drawings:

FIGURE 1 is a diagrammatic representation of a drilling system employed in carrying out the invention.

FIGURE 2 is a graphic illustration of the bottomhole pressure conditions existing during the carrying out of the invention.

FIGURE 3 is a view in section of one type drilling tool which may be used to effect cavitation in a stream of drilling fiuid.

Referring to FIGURE 1, borehole 10 is drilled from the surface of the earth to the desired depth by drilling procedures which may comprise in whole or in part the method of the invention. A drill string 11 is suspended through a rotary table from a swivel in a conventionalmanner, the rotary table and swivel not being shown inasmuch as they are standard elements on all rotary drilling rigs. Supported at the lower end or" the drill string is a drilling tool 12 provided with a head 13 which may be a standard form of drill bit. Drilling tool 12 may be any one of the various forms of drilling tools provided with an element which may be reciprocated in a cyclical fashion longitudinally within a wellbore such that a state of cavitation may be effected within a body of liquid in which the tool is immersed. Where such a bit and drilling tool are used, the intent is to superimpose the cavitational action on the actual physcial contact effected by the drilling bit. If it is desired to rely solely upon cavitational action for drilling the borehead, a tool of the nature illustrated in FIGURE 3 may be employed. The details of the drilling tool of FIGURE 3 will be described hereinafter. Connected into the upper end of drill string 11 are a drilling fluid conduit 14 and a gas conduit 15. In actual operation, conduits 14 and 15 may be interconnected with the swivel in the manner illustrated in U.S. Patent No. 2,082,329, issued to E. V. Foran et al. on June 1, 1937. An arrangement as illustrated in the Foran et al. patent will readily permit drilling fluid with the desired gas added to be introduced into the drill string. Conduit 20 for conducting drilling fluid and gas from the wellbore is connected into the annular space 21 within the wellbore around the drill string.

Referring specifically to FIGURE 3, there is shown a fiuid-turbine-actuated type of reciprocating drilling tool which may be used where it is desired to drill a borehole solely by fiuid action including cavitation. The outer casing is securable to the lower end of a drill string. Positioned within the casing 30 supported on a bearing 31 is a multielement fluid turbine rotor 32 to which is connected a shaft 33. Secured on the lower end of shaft 33 is a bevel gear 34 which meshes with a similar bevel gear 35 which is fixed to a crankshaft 40. Crankshaft is mounted on bearings 41. Secured to the chankshaft 49 is a connecting rod 42, the lower end of which is aflixed to slide assembly 43. The gears, crankshaft, connecting rod, and associated parts are all housed within a casing or gear box 44 which is supported within the casing by a plurality of brackets 45. The brackets 45 are 50 positioned around the gear box that ample space remains within the casing to permit the flow of drilling fluid from the turbine rotor downwardly to the lower portion of the tool. Secured to the lower end of slide assembly 43 is a piston rod 50. A fluid-tight seal 51 is positioned around the piston rod at the lower end of the gear box.

A fluid turbine of the type illustrated in FIGURE 3 is shown in greater detail in the previously referred to patent to Bodine. Any fluid turbine which will generate the necessary power may be employed in the application illustrated in FIGURE 3.

Connected to the lower end of piston rod 50 is a piston 52 which extends across and is larger than the lower open end of casing 30. Piston 52 is provided with a plurality of Openings or passageways 53 which function to permit a flow of drilling fluid from within the casing through lower end of the tool and to direct the fluid against the formation being drilled. Secured around the top side of piston 52 and fitting within the lower end of the casing is a skirt 54 which fits in sliding relationship within the lower inside surface of the casing. If it is desired that a portion of the drilling fluid be allowed to bypass the piston, skirt 54 may be provided with one or more openings 55.

Rotation of the turbine rotor 32 is caused by the flow of drilling fluid from the drilling string into the casing 30. The rotation of the turbine is translated into longitudinal motion to effect reciprocation of piston 52 by means of the various gears which cause the turning of the crankshaft, resulting in the reciprocation of the connecting rod 42. The connecting rod through the slide assembly moves the piston rod to effect translation of the motion into the longitudinal reciprocating action of the pistion rod. The drilling fluid, after passing through the turbine rotor, continues downwardly within the outer casing and passes outwardly against the formation through the openings 55 in the piston. The reciprocating action of piston 52 generates acoustic pressure pulses within the drilling fluid which are superimposed on the hydrostatic pressure of the drilling fluid. When used in connection with the method of the present invention, the piston is reciprocated under conditions which effect a state of cavitation within the drilling fluid. The bubbles 50, illustrated in FIGURE 3, denote the state of cavitation induced in drilling fluid by reciprocation of the piston.

The drilling fluid together with the gas which is added to it in accordance with the invention is pumped downwardly through conduit 11 and outwardly through the head of the drilling tool into contact withthe formation being drilled. The drilling fluid returns to the surface through annular space 21 and exits from the annular space through conduit 20.

In FIGURE 1, P represents the pressure at which both the drilling fluid and the gas added to the drilling fluid are introduced into conduits 14 and 15. AP represents the pressure loss within the drilling fluid and gas in the drill string due to the friction along the inner surface of the drill string. P represents the pressure within the drilling fluid at the bottom of the borehole. AP represents the loss in pressure within the drilling fluid and gas in annular space 21 due to the friction of the surfaces forming the annular space, namely, the inner wall of the borehole and the outer surface of the drill string. P represents the pressure within the combined drilling fluid and gas as it exits from the annular space through conduit 26. L represents the depth of the borehole.

In those cavitational methods which have been suggested before, it has been generally assumed that the liquid drilling fluid in which the cavitational effect is induced at the bottom of the borehole is substantially gasfree or at any rate would generally contain no more gas than would normally be absorbed by the drilling fluid under atmospheric conditions at the surface. In order to establish a state of cavitation in the drilling fluid, the reciprocating portion of the drilling tool must reduce the pressure within the drilling fluid to the point where bubbles or cavities will appear in the drilling fluid. In a substantiall gas-free system, the velocity which must be attained by the reciprocating element of the drilling tool in order to induce a given acoustic pressure is indicated in accordance with the following formula:

where v=the maximum velocity of the reciprocating element,

3=the dynamic pressure (acoustic pressure) within the drilling fluid around the reciprocating element,

g=the acceleration produced by gravity,

=the density of the drilling fluid, and

C=the speed of sound in the drilling fluid.

Presently known apparatus which may be used in drilling to effect cavitation generally has a reciprocable element which is capable of attaining a maximum velocity of approximately five feet per second. Assuming that the drilling fluid to be used is water, the approximate maximum depth at which cavitation could normally be achieved with known drilling tools is determined as follows:

Translating the pressure at which the drilling fluid will cavitate into borehole depth, it can readily be seen that presently available tools generally will not cavitate at a depth in excess of approximately 800 feet where a substantially gas-free water is employed as the drilling fluid. By practicing the method of the invention, this depth may be appreciably extended because of the fact that the drilling fluid will be treated to make it more readily cavitated.

In accordance with the method of the invention, gas is added to the drilling fluid through conduit 15. The pressure P; at which both the drilling fluid and the gas added to the drilling fluid are introduced into conduits 14 and 15 is determined in accordance with the following:

i pf'l ai+ o where AP =the pressure loss due to pipe friction in the fluid flowing through drill string 11,

AP =the pressure loss due to friction in flowing through the annular space 21, and

P =the pressure at which the fluid exits through conduit 20.

The bottomhole pressure P which is the pressure in the zone in which it is desired that cavitation take place, is expressed in the following formula:

bh i+P1 pf where the previously undefined terms are =the effective density of the fluid column between the surface and the bottom of the hole, and L=the depth of the borehole.

In order to achieve the basic objective of the invention the gas is introduced into the drilling mud in a quantity which will saturate the drilling liquid at a pressure level slightly greater than the pressure within the bottom of the borehole at the period during the cycle of operation of the drilling tool when the bottomhole pressure is at its minimum value. By so saturating the drilling fluid with the gas and reducing the pressure at the bottom of the hole to a level slightly below that at which the drilling fluid is saturated, the gas within the drilling fluid will come out of solution to form a plurality of cavities in the vicinity of the bottom of the hole .The graph in FIG- URE 2 shows a curve representing the cyclical operation of the drilling tool superimposed on a line representing the normal bottomhole pressure. The tool while reciprocating increases and decreases the pressure Within its zone of influence in accordance with the waveform shown in FIGURE 2. ii/hen the reciprocating element of the drilling tool is at the bottom of its travel path, it will momentarily increase the pressure within the bottom of the borehole by an amount represented in FIGURE 2 as When the reciprocating element of the drilling tool is at the other end of its travel path such that it is not compressing the liquid below it but rather is retracting from the liquid between it and the bottom of the hole, there is a momentary reduction in pressure by the amount represented in FIGURE 2 as 5. By way of example, assume that the static pressure at the bottom of the hole, P is 1000 pounds per square inch and that the head of the drilling tool 12 is capable of generating an acoustic pressure within the liquid of 300 pounds per square inch, then the maximum pressure during operation of the tool at the bottom of the borehole would be approximately 1300 pounds per square inch while the minimum pressure would be approximately 700 pounds per square inch. It is desired that the state of cavitation develop as near as practicable to the point of minimum pressure Within the borehole bottom. In FIGURE 2, the line marked P represents the pressure level at which the drilling fluid is saturated with the gas under bottomhole conditions. It will be noted that the pressure P is slightly in excess of the value of the bottomhole pressure reduced by the acoustic pressure p. Stated otherwise, the objective is to put a quantity of gas into the drilling fluid which will, at the bottomhole conditions, cause the drilling fluid to be saturated at a pressure slightly in excess of the minimum pressure attainable at the bottom of the hole during operation of the drilling tool. In this way, when the drilling tool has reduced the pressure to the minimum, which is slightly below the pressure at which the drilling fluid was saturated, the gas in the drilling fluid will come out of solution to produce a state of cavitation represented by a plurality of bubbles of gas. In the examples of pressures given above, the pressure at which it would be desired that the drilling fluid be saturated would be at some value equal to or slightly greater than the minimum pressure of 700 pounds per square inch obtained during the cyclical operation of the drilling tool.

It is necessary that a determination be made of the quantity of gas which will result in the drilling fluids being saturated at a pressure level equal to or greater than the average or static bottomhole pressure reduced by the acoustic pressure obtainable with the drilling tool. The amount of gas to be combined with the drilling fluid may be determined in accordance with the formula:

V s2 brp) where V =the volume of gas, standard temperature and pressure of 0 C. and 1 atmosphere pressure, to be added to the drilling fluid through conduit 15, V =the volume of drilling fluid to which the gas is added, a=an absorption coeflicient representing the volume of gas at 0 C. and 760 mm. pressure absorbed by one volume of drilling fluid per unit of applied pressure at the prevailing bottomhole liquid temperature,

P pressure at which saturation of the drilling fluid by the added gas is desired,

P =the static or average bottomhole pressure, and

gr-the acoustic pressure developed by the drilling tool. By way of example, assume the following conditions:

Therefore, 2.7 cubic feet of nitrogn at standard tempera ture and pressure of 0 C. and 1 atmosphere is required to be compressed to the pressure P and injected along with each cubic foot of the water drilling fluid. Assuming the injection pressure P to be atmospheres, then aXP,,=0.01(200SO)=2.7

1 m.027 011. ft.

of nitrogen at the compressed pressure of 100 atmospheres is to be injected through conduit 15 along with each cubic foot of water entering the drill string through conduit 14.

The nitrogen introduced into the water drilling fluid in accordance with the above example dissolves in the water, causing the water to be saturated at the pressure P With the drilling tool operating, the pressure level at the bottom of the hole will alternately be reduced to a level equal to or below the pressure P at which point the nitrogen will cease to remain dissolved in the water. The nitrogen will then come out of solution to form a series of bubbles, establishing the desired condition of cavitation. As the bubbles collapse at the surface or in the pores of the formation through which the borehole is being drilled, high temperatures and pressures result, assisting in the destruction of the formation material and advancing the borehole.

The drilling fluid returns to the surface through annular space 21 and flows from the well through conduit 20 to facilities for treating it, such as removing the cuttings and otherwise preparing the drilling fluid for reinjection into the well. More than likely, the treatment to which the drilling fluid is subjected will allow escape of the gas which had been introduced to aid in cavitation. Therefore, when the drilling fluid is returned to the system through conduit 14, it will be necessary to continue to add gas to the drilling fluid through conduit 15. It will be evident that since the quantity of gas added to aid in cavitation is directly dependent on the depth at which cavitation is calculated to take place, it will be necessary that the quantity of gas injected be increased as the borehole is deepened.

While it is not intended to limit the type of gas which is employed in the invention, it is preferred that a sparingly soluble gas be employed, that is, one which is very slightly soluble in the drilling fluid as distinguished from a gas which will dissolve in large quantities in the drilling fluid. For example, in aqueous drilling fluids, gases such as air, nitrogen, oxygen, methane, or other petroleum gases have been found to be suitable. By aqueous fluid is meant water and drilling fluids which consist of a water base in combination with dissolved and dispersed materials as needed to provide the desired fluid properties. This may also include substantial quantities of solid material in varied states of subdivision, such as derived from the cuttings and chips from the drilling process. Also, mixtures of gases may be employed such as a flue gas comprising in major proportion nitrogen and carbon dioxide. The carbon dioxide, in some instances, may be separated from the other constituents of the flue gas to minimize corrosion problems. Some drilling fluids are of suflicient alkalinity to react with the carbon dioxide to convert it to a soluble electrolyte form which will not, under typical conditions, be reversibly available in gaseous form for the purposes of the invention. Nitrogen and other inert gaseous constituents are not grossly affected by this action. On the other hand, it is preferable to avoid high acidic or alkaline gases such as hydrogen chloride, ammonia, hydrogen sulfide, etc. While no exact line of demarcation can be made between the solubility of those gases which are suitable and unsuitable, obviously it is not desirable to use one such as hydrogen chloride which is known to dissolve in such amounts that thirty percent by weight of the liquid phase could actually be the dissolved gas. A further requirement for the gas used is that it will not be one which will detrimentally alter the chemical structure of the drilling fluid. For example, the gas should not render the drilling fluid acidic when acidic conditions are not desired or cannot be tolerated within the wellbore. Further, the gas should be one which will be above its critical temperature at bottomhole conditions; otherwise liquefaction conditions are remote.

Having thus described the invention, it is intended that such invention shall be limited only within the scope of the following claims.

What is claimed is:

1. In a method of drilling a borehole wherein a drilling fluid is circulated to the bottom of said borehole through a drill string and returned to the surface through the annular space around the drill string and a drilling tool supported at the lower end of the drill string is actuated to induce a state of cavitation in said drilling fluid in the vicinity of the bottom of said borehole, the improvement which comprises the steps of:

(a) introducing into said drilling fluid a. gas in suflicient quantity to substantially saturate said drilling fluid at bottomhole temperature and at a pressure equal to the pressure within said drilling fluid at the bottom of said borehole reduced by the acoustical pressure developed by said drilling tool; and

(b) actuating said drilling tool to alternately reduce the pressure within said drilling fluid at the bottom of said borehole to a value below the saturation pressure for the drilling fluid and gas mixture at the bottom of said borehole whereby said gas is caused to come out of solution to effect a condition of cavitation.

2. The method of claim 1 wherein said gas is sparingly soluble in said drilling fluid.

3. The method of claim 1 wherein said drilling fluid is an aqueous liquid and said gas is air.

4. The method of claim 1 wherein said drilling fluid is an aqueous liquid and said gas is nitrogen.

5. The method of claim 1 wherein said drilling fluid is an aqueous liquid and said gas is oxygen.

6. The method of claim 1 wherein said drilling fluid is an aqueous liquid and said gas is methane.

7. The method of claim 1 wherein said drilling fluid is an aqueous liquid and said gas is flue gas comprising in major proportion nitrogen and carbon dioxide.

8. The method of claim 1 wherein the quantity of said gas is indicated in accordance with the formula:

Where V is the volume of gas at 0 C. and 760 mm. pressure;

V is the volume of drilling fluid;

0c is an absorption coeflicient of the gas-liquid system under bottomhole conditions;

P is the pressure at which saturation of the drilling fluid by the added gas is desired;

P is the initial saturation pressure of gas at surface conditions;

P is the pressure at the bottom of the hole; and

13 is the acoustic pressure developed by the vibrating device.

References Cited UNITED STATES PATENTS 2,082,329 6/1937 Foran et al -48 2,554,005 5/1951 Bodine 175-56 X 2,816,612 12/1957 Hutchison et al 166-177 2,818,230 12/1957 Davis 175-69 3,016,093 1/1962 Bodine 175--56 X 3,045,749 7/1962 Brandon 166177 X CHARLES E. OCONNELL, Primary Examiner.

DAVID H. BROWN, Examiner. 

1. IN A METHOD OF DRILLING A BOREBOLE WHEREIN A DRILLING FLUID IS CIRCULATED TO THE BOTTOM OF SAID BOREH LE THROUGH A DRILL STRING AND RETURNED TO THE SURFACE THROUGH THE ANNULAR SPACE AROUND THE DRILL STRING AND A DRILLING TOOL SUPPORTED AT THE LOWER END OF THE DRILL STRING IS ACTUATED TO INDUCE A STATE OF CAVITATION IN SAID DRILLING FLUID IN THE VICINITY OF THE BOTTOM OF SAID BOREHOLE, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF: (A) INTRODUCING INTO SAID DRILLING FLUID A GAS IN SUFFICIENT QUANTITY TO SUBSTANTIALLY SATURATE SAID DRILLING FLUID AT BOTTOMHOLE TEMPERATURE AND AT A PRESSURE EQUAL TO THE PRESSURE WITHIN SAID DRILLING FLUID AT THE BOTTOM OF SAID BOREHOLE REDUCED BY THE ACOUSTICAL PRESSURE DEVELOPED BY SAID DRILLING TOOL; AND 